The present invention pertains generally to medical tubing and medical devices incorporating such tubing. More specifically, the present invention pertains to medical tubing and corresponding medical devices adapted for percutaneous transluminal use, such as guide catheters, diagnostic catheters such as illustrated in U.S. Pat. No. 5,403,292, and balloon catheters such as illustrated in U.S. Pat. No. 4,762,129. Medical tubing of the present invention is particularly useful to structurally define the lumen of a catheter, e.g., a rapid-exchange balloon catheter or an over-the-wire catheter. The tubing of the present invention is also useful as an inner member in a stent delivery device.
Intravascular catheters are presently in wide clinical use for a variety of diagnostic and therapeutic purposes. Intravascular catheterization therapies, such as angioplasty, atherectomy, and laser irradiation, have been developed as alternatives to bypass surgery for treating vascular diseases or other conditions that occlude or reduce the lumen size of portions of a patient""s vascular system. In particular, balloon angioplasty has proven to be a useful, and in many circumstances, a preferred treatment for obstructive coronary diseases. Also, intravascular diagnostic catheters for angiographics, ultrasonic imaging, and Doppler blood flow measurements for example, have been developed to measure or image the extent of the occlusion of a vessel, (e.g., stenosis). These intravascular diagnostic catheters may be used in conjunction with the aforementioned therapeutic catheters or may be used in conjunction with more invasive techniques such as coronary surgery. Intravascular therapeutic and diagnostic catheters have achieved acceptance because of their effectiveness as well as the fact that their use typically involves a relatively minor surgical procedure as compared to coronary bypass surgery.
However, the effectiveness of the techniques employing these catheters may at times be dependent upon the positioning of the catheter into the vascular system of a patient via an incision at an accessible location which may be remote from the site of occlusion or stenosis. Typically, for example, the intravascular catheter may be introduced into the femoral artery through an incision at the groin and then advanced through the femoral artery to the desired distal coronary site. Because of the small size of some of these vessels and the tortuous passages through the vessels, positioning of a catheter through a patient""s vasculature can be a difficult and time consuming task. Furthermore, the catheters must be able to traverse these tortuous pathways in a manner as atraumatic to the patient as possible. Therefore, in order to limit insertion time and discomfort to the patient, intravascular catheters will preferably have several performance characteristics.
First of all, an intravascular catheter should exhibit good torque control such that manipulation of a proximal portion of the catheter is responsively translated to the tip or distal portion of the catheter. Moreover, the catheter should have sufficient strength in the longitudinal direction so as not to kink or fold as it is advanced through the vascular system. Also, for some types of intravascular catheters, it is desirable to maximize the inner diameter relative to the outer diameter, i.e., to make the lumen as large as practically possible. Specifically, for example, diagnostic catheters generally possess a relatively large lumen to allow fluids, such as radiopaque contrast fluid, to be injected therethrough and out the distal end so that the area of the vascular system under investigation can be viewed fluoroscopically.
Additionally, if the catheter is a dilation catheter, the outer surface of the tubing to be used in an intravascular catheter must be bondable to balloon material. Although the tubing may be bonded to the balloon with adhesive, this is not optimal as the adhesive may fail. Additionally, the adhesive undesirably adds to the surface profile of the catheter. Thus, it is preferable that the outer surface of the tubing of the catheter be directly bondable to the balloon material, such as by fusion bonding, described in U.S. Pat. Nos. 5,501,759 and 5,267,959.
Finally, catheter balloons are now being inflated to higher pressures than has been previously conventional in the art. For example, until recently, balloon inflation pressures typically averaged approximately 12 atmospheres. However, one current trend involves inflating balloons to pressures as high as 28 atmospheres. This relatively high pressure tends to stretch and constrict tubing if the tubing is too weak. In severe cases, the tubing could rupture. Thus, in order to be useful in a balloon catheter involving higher pressures, the tubing must be strong enough to withstand this higher pressure without collapsing or rupturing.
The internal lumen surface of intravascular catheters is subject to performance demands as well. For example, an important function of the internal lumen surface of intravascular catheters is to provide very low surface friction between the catheter and a guidewire and/or treatment device slidably engaging the lumen surface. The low friction internal surface facilitates advancement of the catheter over the guidewire or the advancement of the treatment device through the catheter lumen, as the case may be. Lubricity is especially critical in the curved portion of guide catheters. The low friction internal surface has typically been provided by the use of a lubricious polymer, e.g., polytetrafluoroethylene or the like, as the internal surface material, or alternatively, by coating the internal lumen surface of the catheter with a friction reducing material, such as liquid silicone.
In sum, catheter tubing should possess a combination of the desired characteristics of strength, pushability, torqueability, bondability and lubricity. However, such a combination of characteristics has not been achieved satisfactorily with tubing comprising only a single material. First of all, medical tubing formed from an inherently lubricious polymer tends to be difficult to effectively bond to the material of conventional balloons due to the chemical incompatibility between the materials to be bonded. On the other hand, polymer materials that demonstrate good bonding characteristics with balloons typically must be coated with a lubricant on the interior surface so that the interior surface is sufficiently lubricious, necessitating an additional manufacturing step. Furthermore, such lubricants tend to wear off, so that lubricity is diminished over time.
The prior art also describes several attempts to provide the desired characteristics by utilizing multilayered tubing in intravascular catheters. Conventionally, such multilayered tubing comprises an outer layer of a bondable material such as nylon, polyethylene, polyurethane, or poly(ethylene terephthalate) and an inner layer of a lubricious material such as polytetrafluoroethylene (PTFE) or other lubricious polymer, e.g., high density polyethylene. For example, U.S. Pat. No. 5,538,510 describes a coextrudable, flexible tubing which comprises an outer layer and an inner layer, the two layers being different materials and being covalently bonded to each other. Specifically, the patent purports to provide a length of tubing with the desired combination of properties by using a lubricious polymer as the inner layer, and a stiff polymer as the outer layer. The patent discloses that the flexible tubing is coextrudable and, furthermore, that the lumen of the tubing is sufficiently lubricious so as to obviate the use of a separate low friction sleeve and/or coating. Additionally, U.S. Pat. No. 4,707,389 describes a multi-layered tube composed of an outer layer of ethylenevinylacetate (EVA) and an inner layer of polyvinychloride (PVC), bonded together by a bonding layer. Finally, U.S. Pat. No. 3,561,493 discloses a multi-layered tubing in which the inner and outer layers are welded together by a precompounded layer of the two different polymers.
Although each of these patents purport to provide tubing and/or medical devices with the desired characteristics, problems still remain with existing multilayer tubing structures. For example, the low friction polymeric materials capable of providing a sufficiently lubricious lumen are generally chemically incompatible with the polymeric materials that are capable of providing adequate performance as the catheter outer layer. As a result of this chemical incompatibility, these different classes of materials do not form significant bonds with each other, even upon coextrusion, and thus, tubing comprising layers of these dissimilar materials tends to be subject to delamination. Further, substantial differences between the mechanical properties of the two classes of polymer materials further exacerbates this incompatibility problem.
There is thus a need in the art for medical tubing and medical devices incorporating such tubing that exhibit the desired characteristics of strength, pushability, torqueability, bondability and lumen lubricity. These and other objects are accomplished by the present invention, as hereinafter described.
According to the present invention, the above objectives and other objectives apparent to those skilled in the art upon reading this disclosure are attained by the present invention which is drawn to trilayered tubing as well as to a medical device suitable for percutaneous transluminal use comprising the tubing. More specifically, it is an object of the present invention to provide coextruded, flexible, trilayered tubing, wherein the three layers are firmly bonded together such that the layers resist delamination under operating conditions both normal and extreme (e.g., high balloon pressures of up to 28 atmospheres or more) and furthermore, wherein the materials that comprise the three layers provide the tubing with the desirable characteristics for tubing that is to be used in a medical device suitable for percutaneous transluminal use.
Generally, the present invention provides a length of coextruded, flexible tubing that meets the needs and objectives described hereinabove, by virtue of a multilayer structure. Specifically, the multilayer structure comprises a core layer of a lubricious polymeric material, an outer layer comprising directly bondable (defined below) polymer, and an intermediate tie layer comprising a polymer having pendant functionality capable of adhering the lubricious material of the core layer to the directly bondable material of the outer layer. In this manner, the intermediate tie layer provides a strong connection between the core layer and the outer layer.
In preferred embodiments, the glass transition temperature (Tg) characteristics of the intermediate tie layer are selected to be inbetween those of the core layer and the outer layer. Specifically, it is preferred that the glass transition temperatures vary only gradually from the core layer to the outer layer in order to provide a stage-wise transition of mechanical characteristics from the material of the outer layer to the material of the core layer. Preferably, the glass transition temperature of each layer will be from 85% to 115% of the glass transition temperature of the material(s) adjacent to it. By providing a gradient in the Tg from the core layer to the outer layer, a more stable, more compatible, trilayered tubing is provided that possesses the desired characteristics of strength, pushability, torqueability, bondability, and a lubricious lumen, while also demonstrating dramatically improved resistance against delamination.
The present invention thus provides a length of coextruded, flexible tubing comprising an outer layer having a first glass transition temperature, an intermediate tie layer having a second glass transition temperature, and a core layer having a third glass transition temperature. Preferably, the first glass transition temperature is greater than the second glass transition temperature, which is preferably greater than the third glass transition temperature. Additionally, it is preferred that the outer layer be comprised of a material that is directly bondable to conventional balloon materials. It is further preferred that the core layer is comprised of a material that is lubricious and that the intermediate tie layer is comprised of a material that comprises functionality capable of adhering to both the material of the outer layer and the material of the core layer.
In another aspect, there is also provided a medical device suitable for percutaneous transluminal use comprising the tubing of the present invention and a radially expansive component operationally coupled to the tubing. For example, the tubing of the present invention may be utilized to define the guidewire lumen of a balloon catheter. More specifically, the trilayer tubing of the present invention may define the guidewire lumen of an over-the-wire catheter, i.e., where the guidewire lumen as defined by the trilayered tubing runs the entire length of the catheter. The tubing of the present invention may also define the guidewire lumen of a rapid exchange catheter, i.e., wherein one end of the guidewire lumen as defined by the tubing of the present invention extends through the distal end of the catheter and the opposite end exits through an outer wall of the catheter. Additionally, the trilayered tubing of the present invention may be utilized to form the inner member of a stent-delivery device, wherein a stent is releasably mounted to the tubing of the present invention.
As used herein, the phrase xe2x80x9cdirect bondxe2x80x9d (or xe2x80x9cdirectly bondablexe2x80x9d) is meant to indicate a bond between two materials that requires no bonding substance, i.e, adhesive, interposed between the materials (or materials that are so bondable). Additionally, the term xe2x80x9clubriciousxe2x80x9d as applied to the materials herein is meant to indicate a material that has a kinetic coefficient of friction (steel on polymer) of less than about 0.5. As used herein, xe2x80x9celastomericxe2x80x9d is meant to indicate that property of a material that allows the material to be stretched to at least twice their original length and to recover its original shape partially or completely after the deforming force has been removed. xe2x80x9cGlass transition temperaturexe2x80x9d or xe2x80x9cTgxe2x80x9d as used herein and as is generally known to those of skill in the art, refers to that temperature at which an amorphous material changes from a brittle vitreous state to a plastic state and may be determined by Differential Scanning Calorimetry (DSC). Finally, as used herein, the phrase xe2x80x9cacid-functionalxe2x80x9d is meant to indicate materials that have pendant acidic functional groups.